1. Field of the Invention
This invention pertains generally to vehicle powertrains and transmissions, and more particularly to a method and apparatus for controlling the operating characteristics of an internal combustion engine coupled to a drive train having a mechanical or electrical continuously variable transmission or a standard automatic transmission.
2. Description of the Background Art
The concept of an engine and a "continuously variable transmission" is a very old concept invented in the 1900's, but the theoretical efficiency of the engine, performance and driveability could never be obtained automatically. This can be seen with reference to the conventional powertrain and transmission shown in FIG. 1 where an internal combustion engine 10 has an output shaft 12 that drives a decoupling/starting clutch or torque converter 14, which is in turn coupled to the input shaft 16 of a continuously variable transmission (CVT) or automatic transmission (AT) 18, which in turn has an output or drive shaft 20 coupled to a final drive wheel 22 (e.g., axle and tire). The deficiencies of such a configuration are caused by the dynamic equation representing the engine/CVT system: ##EQU1## where .alpha..sub.DS =acceleration of the vehicle reflected to the drive shaft, ##EQU2## I.sub.E =engine inertia, I.sub.DS =vehicle inertia at the driveshaft, S.sub.E =engine speed, S.sub.DS =drive shaft speed, T.sub.E =engine torque, T.sub.loss =torque losses, and T.sub.RL =road load torque at the driveshaft. Because the first term -.degree.RI.sub.E S.sub.E and the second term T.sub.E R generally oppose each other, the acceleration of the car and the torque and speed of the engine are difficult to control simultaneously. As a result, the best efficiency and minimum emissions for a gasoline or diesel engine cannot be realized without a sacrifice in performance. This can be seen with further reference to FIG. 2 and FIG. 3 which show operating characteristics of the engine as a function of engine speed and torque, where WOT=wide open throttle and denotes the maximum torque line, IOL=ideal torque/speed operating line and denotes where the best efficiency and/or least emissions (minimum brake specific fuel consumption or BSFC) occurs, and POL=practical operating line due to engine/transmission characteristics. Note in FIG. 3 that point A is less efficient than point B but must be used to provide proper vehicle behavior (transient performance).